Fiber-reinforced composites comprised of carbon fiber or other reinforcing fiber and an epoxy resin or other matrix resin are light in weight and superior in corrosion resistance, durability, and other mechanical properties and for example are used in a broad range of fields from for example fishing rods, golf club shafts, and other general use applications to general industrial applications and further to aircraft and other applications etc.
As the method for molding such fiber-reinforced composites, in particular fiber-reinforced composites using carbon fiber as reinforcing fiber, the method of curing and molding an intermediate material comprised of so-called “prepregs” consisting of reinforcing fiber impregnated with a matrix resin is most broadly used.
As the matrix resin when making prepregs, an epoxy resin features various superior properties and therefore is used for fiber-reinforced composites for a broad range of applications from sports applications to industrial applications. As an epoxy resin, conventionally a bisphenol A type epoxy resin has mainly been used, but demand for flame retardant epoxy resins has been rising in recent years for interior applications of aircraft.
As methods for making epoxy resins flame retardant, there are known the 1) method of adding a halogenated epoxy resin (for example, a bromium-modified epoxy resin) to the matrix resin or the method of blending a halogenated epoxy resin (for example, a bromium-modified epoxy resin) and antimony trioxide into the matrix resin (Japanese Unexamined Patent Publication (Kokai) No. 55-92757 and Japanese Unexamined Patent Publication (Kokai) No. p9-278914), 2) the method of adding a flame retardant comprised of a phosphorus-based compound to the matrix resin (Japanese Examined Patent Publication (Kokoku) No. 59-49942), 3) the method of blending aluminum hydroxide or another inorganic filler into the matrix resin (Japanese Unexamined Patent Publication (Kokai) No. 1-197554, and 4) the method of using together an inorganic filler and phosphorus-based flame retardant and blending them into the matrix resin (Japanese Unexamined Patent Publication (Kokai) No. 2000-154301, Japanese Unexamined Patent Publication (Kokai) No. 2000-159980), etc. The methods of imparting flame retardance according to these means 1) to 4) block off the air to stop the supply of oxygen and suppress burning by the halogen gas or steam etc. caused at the time of combustion.
However, fiber-reinforced composites made flame retardant by the means of 1) to 2) have the defects of low heat resistance of the fiber-reinforced composites themselves, brittle mechanical properties, etc. With composites made by the means of 3), the viscosity of the impregnated resin becomes high if trying to impart flame retardance by just an inorganic filler and therefore the impregnatability of the impregnated resin at the time of making prepregs becomes poor. Further, with composites made by the means of 4), the amount of inorganic filler added is large and the viscosity of the impregnated resin becomes high, so not only does the impregnatability of the impregnated resin become poor at the time of making prepregs, but also the tackiness of the prepreg surface ends up being lost and therefore prepregs will not stick to each other or will not stick to the molds when molding prepregs by the hand layup method or other problems of poor moldability will arise. Further, as the ratio of composition of the inorganic component increases, there is also the problem that the mechanical properties or heat resistance of the fiber-reinforced composites obtained will greatly fall.
Further, Japanese Unexamined Patent Publication (Kokai) No. 11-60689 mentions the method of using an epoxy resin obtained by reacting a bisphenol A type epoxy resin, a phenolic novolak type epoxy resin, and flame retardant having functional groups able to react with an epoxy resin, while Japanese National Disclosure Publication (Kohyo) No. 8-507811 shows the method of using a phosphorus-modified epoxy resin comprised of a reaction product of a polyepoxy compound and anhydrous phosphinic acid etc. Composites obtained from these means, however, require a separate process of reacting the epoxy resins with a flame retardant, anhydrous phosphinic acid, etc., so are inconvenient in mass production and therefore are more expensive in price.